1. Field of the Invention
The present invention relates to a cobalt-containing ferromagnetic iron oxide powder having improved dispersibility particularly in an organic binder and very useful as a recording material for magnetic recording media.
2. Description of the Prior Art
Cobalt-containing ferromagnetic iron oxide is characterized by having a high coercive force, enabling high density recording when used as a recording element of magnetic recording media and having an excellent sensitivity in a high frequency range. Therefore, said iron oxide is in wide use in the field of magnetic recording media such as videotape and the like.
As a recent trend, with the use of magnetic recording media (e.g. videotape) of higher quality, the cobalt-containing ferromagnetic iron oxide used in such recording media as a recording element is moving toward finer particles. The magnetic powders of fine particles produced according to the conventional processes have problems in that as the particles become finer, uniform dispersion of the particles in an organic binder when the particles are kneaded with the binder to make a magnetic coating becomes more difficult and resultantly the intended purpose of obtaining a magnetic recording medium of improved magnetic characteristics by using a magnetic powder of finer particles cannot be achieved.
In the recent production of magnetic recording media, it is important in order to allow such media to have excellent magnetic characteristics that a magnetic powder of high coercive force be highly packed and highly oriented in the recording media, or a magnetic coating containing a uniformly dispersed magnetic powder be used to allow the magnetic recording media produced therefrom to have a surface of high smoothness. In order to achieve these requirements, it is necessary that the magnetic powder be disintegrated into nearly its primary particles without destroying the powder particles when a magnetic coating is prepared from the powder. With the magnetic powders produced according to the conventional processes, however, the primary particles tend to aggregate to form agglomerates and thus disintegration of the powders into nearly their primary particles at the time of magnetic coating preparation is difficult.
In order to solve the above problems, various proposals have been made. For example, there have been tried an approach of coating the particle surfaces of a magnetic powder with, for example, a surfactant having good compatibility with a binder used together with the powder, prior to the preparation of a magnetic coating from the powder and the binder [Japanese Patent Kokoku (Post-Exam. Publn.) No. 19120/1978 and Japanese Patent Kokai (Laid-Open) Nos. 37297/1979, 141196/1978, 82354/1979 and 85397/1979], an approach of using a surfactant as a dispersing agent at the time of preparation of a magnetic coating [Japanese Patent Kokai (Laid-Open) Nos. 151068/1980 and 151069/1980] and an approach of disintegrating agglomerates of magnetic powder particles using a mechanical means, at the time of preparation of a magnetic coating [Japanese Patent Kokai (Laid-Open) Nos. 22297/1975, 157216/1980 and 10903/1981].
However, these approaches have drawbacks in that the surfactant is not effective for its selectivity for binder resin, in that the magnetic tapes produced have a reduced strength or cause bleeding or fall-off of powder (which means a phenomenon wherein the magnetic powder contained in the tapes is peeled off when the tapes are rubbed), and in that the particles obtained by mechanical dispersion cause reaggregation depending upon the degree of the dispersion. These drawbacks are larger when the particles of magnetic powder are finer, making very difficult the improvement of dispersibility of finer particle magnetic powder.
The processes for producing a cobalt-containing ferromagnetic iron oxide powder by allowing a cobalt compound to adhere to the particle surfaces of a magnetic iron oxide powder include, after the adhesion of the cobalt compound, a drying step using a dryer. This dryer is ordinarily a through flow chamber dryer, a fluidized bed dryer, a through flow rotary dryer, etc. To this date, there has been made no attempt to enhance the dispersibility of a magnetic iron oxide powder through the improvement of the drying step. The present inventors made extensive research in order to solve the previously mentioned drawbacks of the conventional processes for producing a cobalt-containing iron oxide powder and, as a result, found that the use of a spray dryer in the above drying step enables the production of a cobalt-containing ferromagnetic iron oxide powder of very excellent dispersibility whose particles are each an aggregate of cobalt-containing ferromagnetic iron oxide microparticles and has a spherical shape of 5 to 200 .mu.m in diameter and which can be easily disintegrated, at the time of production of a magnetic recording medium, into nearly the state of the primary particles before aggregation. This has led to the completion of the present invention.